The present invention relates to packaging films. Specifically, the present invention relates to multiple layer heat sealable packaging films.
Biaxially oriented polyethylene terephtlatate, commonly referred to as "oriented polyester" or "OPET", is a common packaging film with over 200 million lbs. used annually. However, an important criteria for any packaging film is its ability to be heat sealed to itself as well as to other substrates. OPET films are not heat sealable. In recent years, however, heat sealable OPET has become commercially available incorporating amorphous homopolymers or copolymers of PET as heat sealants applied either as a coating or coextrusion. OL Mylar is an example of a heat sealable OPET film having the heat sealants applied as a coating. OL Mylar is a registered trademark of E.I. DuPont de Nemours and Company. Melinex 850 is an example of a heat sealable OPET film having the heat sealants applied as a coextrusion. Melinex is a registered trademark of Imperial Chemical Industries, Ltd.
Recently, heat sealable OPET films have been used as the inside layer of a paper-film laminated bag for cooking popcorn and other foodstuffs in a microwave oven. Microwave popcorn bags are described in U.S. Pat. Nos. 3,973,045 and 4,450,180. However, the bags of these inventions do not contain an inside film liner, rather they consist of two plys of paper. Typically, the outer ply is a bleached Kraft paper, while the inner ply is a greaseproof paper such as glassine. These bags are intended for a frozen product in which the bag is filled with the popcorn cooking oil mixture and then frozen. The product is shipped and stored frozen.
In recent years, "shelf stable" products such as microwave popcorn bags have been introduced. These products do not require any refrigeration. However, the greaseproof paper lined bag used for the frozen product was not adequate for the shelf stable product in that the cooking oil in the unfrozen state would leak through and stain the outer paper liner. This problem was eliminated by changing the inner ply to a plastic film. In particular, heat sealable OPET films are used as the inner ply of the shelf stable bag. An example of this product is Orville Redenbacher's Gourmet Microwave Popping Corn. Orville Redenbacher's and Gourmet are registered trademarks of the Beatrice Companies, Inc.
OPET films are manufactured by an orientation process utilizing tenter frame or "double bubble" techniques. Tentering of plastic films is illustrated in U.S. Pat. No. 2,823,421 and double-bubble orientation is illustrated in U.S. Pat. No. 3,555,604. U.S. Pat. Nos. 4,141,736 and 4,207,363 describes oriented films made using the tenter and double bubble processes, respectively. The orientation process comprises the steps of (1) extruding a thick web and cooling to a solid state; (2) precise reheating of film to the "orientation" temperature, (3) stretching the film in both longitudinal and transverse directions, and (4) reheating film to relieve internal stresses. Melinex 850 comprises two layers coextruded in step (1) of the orientation process prior to reheating to the orientation temperature. An example of a coextruded oriented PET is disclosed in U.S. Pat. No. 4,375,494. This heat sealant oriented PET differs from the films of this invention in that the film is oriented.
Alternatively, monolayer OPET can be made heat sealable by laminating it to a heat sealant film. This lamination is done in a separate manufacturing operation. Typically the heat sealant is an olefinic film such as polyethylene. An example of such a lamination is disclosed in U.S. Pat. No. 4,178,401.
The film disclosed herein consists of a base substrate layer "B" coextruded with one or two heat seal surface layers "A" yielding either an "AB" or "ABA" multiple layer film. The film can be produced by conventional blown or cast coextrusion techniques.
Conventional blown film coextrusion techniques and equipment therefor is known in the art and is commercially available. Also, conventional cast film coextrusion techniques and equipment therefor is known in the art and is commercially available. The following U.S. patents disclose various coextrusion extrusion techniques and equipment therefor: U.S. Pat. Nos. 4,484,883; 4,483,812; 4,465,449; 4,405,547; 4,403,934; 3,611,492; 3,559,239; 3,476,627; 3,337,914; 3,223,761 and 3,467,565.
Two conventional cast coextrusion techniques are known in the art. The first method combines the molten polymers in a combining adaptor prior to entering the slot cast die. The second method does not bring the molten polymers in contact with each other, until the polymer melt streams are inside the die. Either method will yield a cast coextruded film with very similar properties.
The base layer "B" comprises polyester and/or copolyester. Polyester is defined herein as poly(ethylene terephthalate) which is typically made by reacting terephthalic acid with ethylene glycol. Other acids or glycols can be used. Copolyesters are made by reacting more than one acid or glycol.
This "B" layer imparts the physical strength of the film, i.e., toughness, tear resistance, stiffness, etc., as well as the thermal resistance. The base layer comprises 50%-99% mil/mil of the total film thickness, preferably 75%-95% mil/mil.
The heat sealant layer "A" comprises a copolyester. This copolyester is not the same copolyester utilized in the "B" layer. This can be utilized in the "B" layer provided same is not utilized in the heat sealant layer. This copolyester will typically have a lower softening/melting temperature than the "B" layer. This property enables the "A" layer to provide strong heat seals over a broad range of temperatures.
The "A" layer promotes a heat seal when the film is heat sealed to (1) the "A" or "B" layer of film, or (2) other films or rigid substrates. The heat seal is accomplished by applying heat and pressure against the film and substrate. An example of a heat seal is that found on packages of snack foods or the bottom of a plastic trash bag. The heat seal can be either peelable or fused.
Optionally, the heat seal layer "A" may contain slip and antiblock agents to lower the coefficient of friction, C.O.F., of the film as well as to prevent roll blocking. Low C.O.F. (0.2-0.5 g) is often necessary to enable the film to "slide through" various converting machinery.
The "A" layer or layers comprises 1%-50% mil/mil of the total film thickness, preferably 5%-25% mil/mil. The composition of the polymers in "A" are a polyester or copolyester resin or blend thereof using compatible polymers.
FIG. 3 illustrates an adhesive lamination of films of the invention to paper.
The total thickness of the film is from 0.5 to 5 mil (0.0005-0.005 inches) preferably 0.5 to 2 mil (0.0005-0.002 inches).
The present invention provides compositions of nonoriented multiple layer heat sealable films which can be used in many packaging applications where heat seal OPET films are used. The nonoriented multiple layer heat sealable films of the present invention are manufactured by conventional cast or blown film nonorienting coextrusion techniques.
Conventional film coextrusion equipment costs substantially less than orientation equipment and is significantly easier and less expensive to operate. Those skilled in the art can appreciate the significance of producing a nonoriented film which can be used to replace an oriented film.
Nonoriented single layer PET films are well known, An example is disclosed in U.S. Pat. No. 4,175,147. This patent discloses single layer nonoriented films comprised of 97.5% to 99.9% w/w of PET blended with 2.5 to 0.1% of polycarbonate. Nonoriented multilayer film wherein one layer includes PET is disclosed in U.S. Pat. No. 4,389,450. This patent discloses a multiple layer heat sealable packaging sheet. The base layer (nonsealant) layer can comprise of a blend of 75% polyester and 25% olefinic material (e.g., ethylene acrylic acid, ethylene methyl acrylate, ethylene ethyl acrylate, ionomer, and the like). The heat sealant layer is selected from the same group of olefinic polymers. The films of this invention differ in that they contain non-olefinic heat sealants (polyesters and copolyesters) and the base layers do not contain any olefinic polymers. Nonoriented heat sealable films are also disclosed in U.S. Pat. No. 4,469,754. This patent discloses olifinic compositions intended for heat sealant layers in coextrusions, coatings and laminations. The benefits of a broad heat seal range in packaging films are discussed.
The present invention demonstrates the following additional advantages over heat sealable OPET films: (1) the films of the present invention have better surface receptivity to inks, adhesives, and the like; (2) the films of the present invention exhibit superior heat seal properties; (3) the films of the present invention are thermoformable; and (4) "AB" films of the present invention are lap sealable to themselves. Such additional advantages are a result of the relatively amorphous nature of the nonoriented films of this invention compared to the relatively crystalline nature of the OPET films.
The present invention provides film compositions which can be used in place of heat sealable OPET film in extremely demanding applications such as microwave popcorn bags.
It is an object of the present invention to provide compositions of nonoriented multiple layer heat sealable films having many of the important properties of heat sealable OPET films.
It is another object of the present invention to provide nonoriented multiple layer heat sealable films manufactured by conventional cast or blown film nonorienting coextrusion techniques.
It is a further object of the present invention to provide nonoriented multiple layer neat sealable films having the following advantages over heat sealable OPET films: (1) better surface receptivity to inks, adhesives and the like; (2) superior heat seal properties; (3) thermoformability; and (4) lap sealability of "AB" films.
It is a further object of the present invention to provide nonoriented multiple layer heat sealable films which can be utilized as an inside liner of a laminate bag such as a microwave popcorn bag produced by laminating the film as the inside plastic film liner to a paper layer and forming the bag from the resulting laminate.
These and other objects of the present invention will be apparent from the description of the preferred embodiments which follows. Such objects are not intended to limit the scope of the present invention.